In one method of forming mineral fibers, such as glass, for example, batch materials or preformed marbles are melted and continuous filaments of molten material are pulled from a bushing. These filaments, known as primaries, are then reheated by an attenuating burner to a temperature in excess of the softening temperature of the glass in order that the filaments may be attenuated. These attenuating burners have extremely high gas flow rates in order to stretch the filaments while they are heated so as to reduce their diameter. As the attenuated filaments cool below the melting temperature of the glass, these filaments are broken by the force of the attenuating blast into fibers within a predetermined range of lengths, this range being a function of the operational parameters and the configuration of the attenuation zone.
As energy costs continue to spiral upward, efforts are being expanded to find ways to reduce the amount of gas used in fiber manufacturing. Of the total energy expended in fiberizing glass, or the like, from raw materials, approximately 2/3 is used in attenuating the fibers. Even a small percentage reduction in the amount of gas used in fiber attenuation can result in significant cost avoidance. The high velocity attenuation blast entrains cooler air from its surroundings. This low energy, low velocity air is mixed with the attenuation stream thereby diluting it and reducing both its temperature and velocity. The capability of the attenuating apparatus to reduce fiber diameter (i.e., to improve the insulating capabilities of the material) is hampered by this unrestricted stream dilution. To offset the disadvantages of dilution, more gas must be burned to produce higher temperatures.
The present invention overcomes the problems of unrestricted dilution by providing a shroud around the attenuation region. This attenuation shroud limits the entrainment of dilution air by restricting the access of the surroundings to the region. The shroud also confines the heat thereby increasing the temperature in the attenuation zone. Several openings are provided in the shroud to permit a restricted amount of dilution air to be beneficially entrained by the attenuation stream. The dilution air from at least one of the openings is provided with a preheater which uses waste heat rising from the attenuating burner to heat the air. This preheated dilution air, preferably enters the shroud from the rear where it performs two useful functions. Firstly, the preheated dilution air strikes the primary filaments and preheats them prior to their exposure to the main attenuating blast. Secondly, this inspirated flow of air provides a downward and forward force which assists in maintaining the primaries in contact with a threaded spacer bar that reduces the frequency with which the filaments become entangled with each other.
The attenuating burner with which this shroud is used has a mounting which permits a degree of play in its mounting. That is, while the position of the front end of the burner is generally fixed, the position of the rear end of the burner may be located in a number of horizontal planes to accommodate attachment to upstream elements, resulting in a variability in the direction of flow of the attenuation blast. A completely stationary shroud could, therefore, find itself in the path of the attenuation blast resulting in fiber buildup, flow disruption and, possibly, damage to the shroud itself.
The shroud of the present invention avoids these problems by being formed of a first stationary portion mounted adjacent the burner and a second downstream portion pivotally mounted to the first portion. In this manner, the forward end of the shroud may be adjusted upwardly or downwardly to more generally be aligned with the flow direction of the blast.
The position of the stream of gases can also be adjusted within the shroud by adjusting the amount of air inspirated above and below the centerline of the blast or stream. A first opening is provided below the centerline of the stream and is located between the burner and the shroud. The size of this first opening is varied as the position of the pivotal shroud portion is adjusted. A second opening is positioned above the centerline of the stream and asymmetrically with respect to the first opening. This second opening is provided with an adjustable closure of damper so that the amount of air inspirated through this opening can be adjusted, preferably, to balance the amount of air inspirated through the first opening, to properly position the attenuation stream. The inspirated air stream from this second opening impinges upon the main attenuation stream at a significant angle and in sufficient flow quantities to create turbulence in the combined stream. This turbulence causes the primary filaments to adopt a serpentine path within the attenuation zone which increases the length of time each primary is exposed to the heat of the attenuation zone and thereby improves fiber attenuation (i.e., reduces fiber diameter).
Other features, advantages and characteristics will become apparent after a reading of the following specification.